The purpose of this work is to characterize the genetic changes that occur during speciation. Our goals are to isolate and characterize gene(s) that cause hybrid sterility and to identify the population genetic forces that drive their divergence between species. Using genetic and molecular population genetic approaches in Drosophila, we will address the following questions: A. Which genes cause hybrid sterility between species? B. Do these genes have normal functions within species? If so, what are these functions? C. Did these genes diverge by positive Darwinian selection or by random genetic drift? D. What is the nature of the molecular differences distinguishing species at these genes? E. Does hybrid sterility, like hybrid inviability, often map to single genes? F. Does meiotic drive in species hybrids cause hybrid sterility? We will attempt to answer most of these questions via analysis of a new hybrid sterility gene, JYAIpha, recently discovered in our laboratory. Our long-term goal is to understand how animal species, including humans, arise by uncovering the genetic causes of speciation.